MYCOTIC DERMATITIS IN DOGS AND CATS:  A REVIEW OF 28 CASES SUBMITTED TO THE INDIANA ANIMAL DISEASE DIAGNOSTIC LABORATORY

Diagnosis of cutaneous fungal infections is often made clini­cally based on the signalmentand gross appearance of the lesion, supported by cytologicpreparations, culture, the use of Dermatophyte Test Medium ,and response to therapy.  In a small percentage of cases, treatment failure, chronic in­fections, or presentation of the lesion in an unusual loca­tion or host, may necessitate the use of histopathology for diagnosis.

A retrospective survey of cases submitted to the Indiana Animal Disease Diagnostic Laboratory from July 1, 1988 to October 1, 1992 was performed to determine what mycotic agents were involved in cutane­ous infections in the dog and cat in which histopathology was requested.  A total of 24 canine and 4 feline cases were selected in which fungal agents had been observed by light mi­croscopy and/or isolated via culture.  Dermatophytosis was the most common mycotic infection observed (20 of 24 dogs, and 2 of 4 cats).  In addition, one case each of sporotrichosis (feline), histoplasmosis (canine), geotrichosis (canine), paecilomycosis (canine) and cryptococcosis (feline) were diagnosed.  Also, although not currently considered a mycotic agent, a single case of canine pythiosis was also included in the study.

Although dermatophytosis(ringworm) is generally considered a disease of young animals,⁶'⁷ many of the affected animals in this study were adults.  Age ranged from10 weeks to 13 years, with a mean of 3.5 years.  In 4 of 22 cases (18.2%) there was evi­dence of a compromised immune system.  In 36.4% of the animals, lesions were described as nodules, plaques, or pedunculated masses rather than areas of alopecia or exudative lesions as is more commonly described.⁶'⁷  Based on the morphology and arrangement of the arthrospores and fungal hyphae,Microsporumcanis was the most common dermatophyteinvolved.  The majority of cases responded to topical or systemic antifungal therapy, or were resolved with surgical excision.

Cryptococcosis is also regarded as an opportunistic infection, and is more common in cats than dogs.

Geotrichumsp. has been a reported cause of bovine masti­tis," and dermatomycosis in cats, but to date, cutaneous infections have not been re­ported in the dog.   The dog in this study presented with a rapidly enlarging skin "tumor". Although cultures were not per­formed on the biopsy, the mor­phologic appearance of the fun­gus was similar to that de­scribed previously for Geotri­chum sp. .'   Geotrichum sp. are considered contaminants that rarely cause disease in healthy individuals;⁴'  howev­er, the demonstration of these fungal hyphae deep within the affected dermis of this dog supports a diagnosis of cutane­ousgeothrichosis.  Details concerning the immune status of this dog, and follow-up infor­mation were not available.

Sporotrichosis was diagnosed in a cat based on the micro­scopic alterations in skin as well as the isolation of the causative agent (Sporothrixschenkeii).  Fungal culture of this agent is considered very reliable, and in dogs where organisms are difficult to demonstrate microscopically, a positive culture and supportive microscopic lesions are used to make a definitive diagnosis.⁶'⁸ In most cases of feline sporo-trichosis, organisms are numer­ous and are easily identified in tissue sections.⁶  Sporotri­chosis is generally observed secondary to contamination of wounds with Sporothrixschen-keii.a saprophyte found in soil, humus and vegetation.  It is considered a transmissible disease, presenting significant health risks to attending vet­erinarians and staff.   Follow-up information obtained from the referring veterinarian re­vealed that this cat is often involved in cat fights, which may have been the entry point for this infection.

Histoplasmosis is primarily a systemic disease that has the respiratory system as the major portal of entry.⁹  Primarycu­taneous lesions, such as in the dog in this study, are rare. Although the dog was eventually lost to follow-up, marked clin­ical improvement was observed with use of systemic corticost-eroids over a 4-month period.

Paecilomycosis has been reported previously in dogs and cats.³'⁵  However, in these re­ports, the infections were ful­minant and inevitably fatal. In contrast, lesions in the dog in this study were reportedly present for approximately 2 years.  Paecilomyces sp. is considered an opportunistic pathogen that causes disease in cats.-

immunocompromised or debilitated hosts.⁴'   No history con­cerning the immune status of this patient, or the final out­come in this case, was avail­able.

Cryptococcosis is also re­garded as an opportunistic infection, and is more common in cats than dogs."  Similar to histoplasmosis, most cutaneous lesions are probably secondary to systemic involvement, although primary cutaneous lesions of the skin of the nose and head have been reported in cats.⁸  Response to therapy is poor.⁶  No further clinical history or outcome was avail­able for the infected cat in this study.

A single case of canine pithyosis was identified.  Al­though the causative agent is no longer classified as a fun­gus- it has been included in this study as it poses a diagn-sotic challenge.  Microscopic lesions of pithyosis are similar to those reported for Basidiobolussp.,Conidibolussp. and other mycotic agents. Pythium sp.,Basidiobolus sp., and Conidibolus sp. do not stain with routine hematoxylinand eosin preparations, a fea­ture which helps to distinguish them from other mycotic agents.   Gomori'smethenaminesilver stain is useful in iden­tification of this organism histologically, but definitive diagnosis requires special cul­tures and the identification of motile zoospores.   In dogs, cutaneouspythiosis is less common than alimentary forms of infection and is roost often reported in young, male, large breed dogs in the southern United States.¹  Both alimen­tary and cutaneous forms of pythiosis have a guarded prog­nosis.  The case in this report involved a spayed, female poo­dle dog, 4.5 years of age.  No follow-up information was available on this case.

In conclusion, cutaneous fun­gal infections may present with a wide variety of clinical signs, and gross and microscop­ic lesions.  Although the histologic lesions, and the morphologic features and staining characteristics of the fungus can be used to diagnose many fungal infections, the use of mycotic cultures may be war­ranted when there is a public health risk (M.canis,Sporo-trichosis) or when a definitive fungal identification (and sen­sitivity pattern to antifungal agents) is desired.

Rachel Y. Reams Rivera,DVM
Daniel D.Harrington, DVM, PhD

LIST OF REFERENCES

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